The exact renormalization group in astrophysics

The coarse-graining; operation in hydrodynamics is equivalent to a change o f scale which can be formalized as a renormalization group transformation. In particular, its application to the probability distribution of a. self-g ravitating fluid yields an "exact renormalization group equation" of Fokker -Planck type. Since the time evolution of that distribution can also be des cribed by a Fokker-Planck equation, we propose a connection between both eq uations, that is, a connection between scale and time evolution. We finally remark on the essentially non-perturbative nature of astrophysical problem s, which suggests that the exact renormalization group is the adequate tool for them.